This invention relates to a method of producing a solid adsorbent for the separation and recovery of CO from any mixed gas containing CO by pressure swing adsorption (hereinafter referred to as PSA) and/or temperature swing adsorption (hereinafter referred to as TSA).
Representative gases containing CO as a main component include converter gas from steelmill converters, blast furnace gas from blast furnaces, electric furnace gas from electric furnaces and producer gas which can be obtained by gasifying coke. Most of these gases have been used as fuels and contain around 70% by volume or more of CO. Therefore, if the CO contained in these gases can be separated or recovered in high purities, it may be utilized as a starting material for synthesizing formic acid, acetic acid, etc., or a reducing agent for reduction of organic compounds, etc., thus affording great benefits to chemical industries.
Heretofore, as processes for the separation and recovery of CO from gases containing CO as a main component, there have been known the low-temperature processing technology, the cuprous ammonium process, COSORB process and so on. These processes involve such disadvantages as high equipment costs and high costs of utilities such as electric power, steam and other thermal energies, so that although they are suitable for the mass separation and recovery of CO in large quantities, they are not necessarily suitable for the separation and recovery of CO on a medium or small scale. Moreover, since the CO obtained by such known methods contain impurities such as O.sub.2, CO.sub.2, etc., which are deleterious to reactions of organic syntheses, it cannot be utilized in organic syntheses.
As methods for accomplishing a selective separation of specific gases from medium or small volumes of material gases, PSA and TSA methods are known.
The PSA method is a method for selective separation of a specified gas from a mixed gas and comprises adsorbing the desired gas on an adsorbent at a high pressure and then, desorbing the adsorbed gas under reduced pressure. In the industrial application of the process, a plurality of columns packed with the adsorbent are installed and in each adsorption column, a series of operations of pressure-up--adsorption--cleaning--degassing is repeated so as to effect a continuous separation and recovery of the product gas as a whole.
The TSA method, like the aforementioned PSA method, is a method for selective separation of a specified gas from a mexed gas. Thus, the desired gas is first absorbed on an adsorbent at a low temperature and then, by raising the temperature of the adsorption system, the adsorbed gas is desorbed.
Heretofore, as a process embodying the principle of this PSA method, a process utilizing mordenitic zeolite as the adsorbent has been proposed. (Japanese Patent Un-Examined Publication No. 22625/1984; ibid. No. 49818/1984.)
Furthermore, for the separation and recovery of CO from a mixed gas containing CO according to the principle of the PSA or TSA method, various processes employing copper compounds, such as cuprous halides, cuprous oxide, cupric salts, cupric oxide, etc., as supported on active carbon have been proposed. (Japanese Patent Un-Examined Publication Nos. 156517/1983, 69414/1984, 105841/1984 and 136134/1984.)
As a method of preparing a CO adsorbent for use in the separation and recovery of CO from mixed gases containing CO by the PSA or TSA method, there has been proposed a process which comprises contacting an organic solvent solution of cuprous halide and alminium halide with a porous inorganic oxide(s) such as alumina, silica or silica/alumina, etc., and then, removing the free organic solvent. (Japanese Patent Un-Examined Publication Nos. 90036/1985 and 90037/1985.)
The applicant of the present invention has already filed a Japanese patent application for a method of producing an adsorbent consisting of a silica and/or alumina carrier and a cuprous compound or a cupric compound or a reduction product thereof as supported thereon for the separation and recovery of CO from mixed gases containing CO by the PSA or TSA method. (Japanese Patent Un-Examined Publication No. 242638/1986)
As the properties required of an adsorbent to be packed into an adsorption column in the practice of the PSA or TSA method, there may be mentioned: (i) it is capable of selective adsorption of the object component to the exclusion of other coexisting components, (ii) there is a large difference in the adsorption rate of the object component between the time when the column is under pressure or at low temperature and the time when the column is under reduced pressure or at high temperature, (iii) removal of the adsorbed object component is easy, (iv) other components are not easily adsorbed and desorbed, and (v) the life span of the adsorbent is long. The above properties are important factors in the PSA or TSA method, for the purity and yield of the product gas are greatly influenced by these properties.
The method utilizing mordenitic zeolite, which takes advantage of physical adsorption-desorption, involves the following problems. Because the inherent adsorption capacity of this adsorbent for CO is relatively small, the frequency of pressure swing must be increased and this results in disadvantages not only in operation but also in the service lives of various valves. Moreover, prior to the adsorption stage, CO.sub.2 must be removed previously. The purity of the product CO gas is low because of concomitant adsorption of N.sub.2 and, furthermore, because a large amount of product CO gas must be used for the cleaning of the inner parts of the column for removal of adsorbed N.sub.2, the yield of the product CO is decreased.
On the other hand, the method using an adsorbent having a copper compound carried on active carbon, which takes advantage of chemical adsorption-desorption, involves the following problems. Thus, when CO is to be separated from a mixed gas containing CO, N.sub.2 CO.sub.2, etc., the CO.sub.2, etc. tend to be co-adsorbed, together with CO and therefore, it is difficult to obtain high purity CO. Moreover, the CO adsorption capacity of the adsorbent is not necessarily large. For these reasons, his method has not yet reached the stage of implementation on an industrial scale.
The method which comprises using an adsorbent having cuprous halide and aluminum halide as supported on a porous inorganic oxide takes advantage mainly of the selective CO adsorbency of CuAlX.sub.4 (X is halogen). However, this method still needs to be improved for industrial application in such respects as that because its adsorbing capacity for CO is too great, it is difficult to remove the adsorbed CO at the step of degassing and the technology is accordingly not suitable particularly for the PSA method. In addition, the preparation of the adsorbent must be performed in a dry inert gas atmosphere and it is difficult to restore the activity of the adsorbent once it has been degraded.
Then, the invention described in Japanese Patent Un-Examined Publication No. 242638/1986 as filed by this applicant, namely the method using an adsorbent comprising a silica or/and alumina carrier and either a cuprous compound or a cupric compound or a reduction product thereof as supported thereon has the advantage that it affords a separation and recovery of CO of extremely high purity but still remains to be improved from industrial points of view.
Thus, when a cuprous compound is chosen as the compound to be supported on a silica or/and alumina carrier, it is commercially common practice to dissolve the cuprous chloride in concentrated hydrochloric acid, treat the carrier with the resulting solution and dry the same. However, the use of concentrated hydrochloric acid involves a high risk of injury to the operator and the preparation, supporting and drying steps must be carried out in an inert gas, thus complicating the production process. Moreover, when hydrochloric acid is used as the solvent, the crushing strength of the carrier itself is sacrificed so that the life of the column packing is shortened. Another disadvantage is that the dispersion within the carrier is poor as compared with the case in which a cupric compound is employed.
On the other hand, when a cupric compound is chosen as the compound to be supported on a silica or/and alumina carrier, it is commercially common practice to use cupric chloride However, when cupric chloride is used alone, the heat treatment for increasing the CO adsorption/desorption capacity of the adsorbent must be carried out at a comparatively high temperature. Furthermore, even if the temperature of the heat treatment is so increased and the heat treatment is further followed by heat treatment in a reducing atmosphere such a CO or H.sub.2, the CO adsorption/desorption capacity is not increased as much as desired.
In view of this situation, the present invention has been accomplished as a result of diligent research pursued in an effort to find out an adsorbent which is capable of making a separation and recovery of high purity CO with high efficiency from any mixed gas containing CO.